Photoluminescence imaging or photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials, such as silicon semiconductor wafers, as well as other workpieces and materials. In a typical photoluminescence process, light is directed onto a wafer or other workpiece or sample (hereinafter collectively referred to as a “wafer”), where some of the light is absorbed. The absorbed light imparts excess energy into the material via a process of “photo-excitation.” This excess energy is dissipated by the wafer through the emission of light, or photoluminescence. The intensity and spectral content of this photoluminescence is directly related to various material properties of the wafer.
Photoluminescence imaging processes may be used to identify and quantify defects and contaminants present in the wafer based on spatial variations in the photoluminescence images produced. One photoluminescence imaging process, as described in International Application Number PCT/GB97/02388 (publication number WO 98/11425), which is incorporated herein by reference, involves probing the surface and/or the sub-surface bulk region of the wafer with one or more lasers of varying excitation wavelengths. A laser of a given wavelength is directed into the wafer and penetrates the wafer to a given depth. Return light emitted from the wafer is detected and quantified by a detection system. Images of the measured return light, including spatial images of defects and contaminants in the wafer, may then be produced by the detection system or by an associated image-producing system.
While these images may effectively identify defects and contaminants in the wafer, they are sometimes distorted due to non-uniformity in the surface layer or epitaxial layer of the wafer. Wafer surface variances may be caused by, for example, a Bernoulli wand or similar device that suspends the wafer and applies moving gas over the wafer's surface. The distorted images resulting from these surface variances can make it difficult or impossible to accurately identify and quantify defects and contaminants in the sub-surface or bulk region of the wafer. Thus, to obtain a more accurate measurement of the defects and contaminants in the bulk region of a wafer, photoluminescence image distortion, caused by variances in the surface layer of the wafer, must be substantially reduced or eliminated.